A vet visit + meds wouldn't be that much, (Depending on your vet and the amount of meds needed it would probably be under $80.) but most vets prescribe either doxycycline or baytril for sniffles/respiratory infections. A cheap way to treat it at home is to buy a product called "Bird Biotic" and put it in the water. It'd be best to ask a vet for the proper dosing, but it's better then doing nothing.

http://www.agmahs.com/product/DOXYTL100

Doxy is safe in young/nursing rats, but it's not a good idea to give it to a pregnant rat.